Atlas of Anatomy. Head and Neuroanatomy. Michael Schuenke

20. Sectional Anatomy of the Brain

20.1 Coronal Sections: I and II (Frontal)

General remarks on sectional brain anatomy

The series of sections (coronal, transverse, and sagittal) in this chapter is intended to help the reader gain an appreciation of the three-dimensional anatomy of the brain. This is necessary for the correct interpretation of modern sectional imaging modalities (CT and MRI for the investigation of suspected stroke, brain tumors, meningitis, and trauma). In offering this synoptic perspective, we assume that the reader has read the previous chapters and has gained at least a general appreciation of the functional and descriptive anatomy of the brain. The legends and especiallythe small accompanying schematic diagrams are intended to facilitate a three-dimensional understanding of the two-dimensional sections (the plane of the section in each figure is indicated by a red line in the small, inset image).

The planes of section have been selected to display the structures of greatest clinical importance more clearly than can be done in actual tissue sections, which are not always optimally fixed and preserved. Because the sections were modeled on specimens taken from different individuals, some structures will not be found at the same location in every figure. The structures of the brain were assigned to specific ontogenetic regions in previous chapters, and these relationships are summarized in B, p. 315, at the end of this chapter.

Note the relationship of the sectional planes to the Forel axis in the anterior part of the brain and to the Meynert axis in the brainstem region (see D, p.185).

A Coronal section I

The body (trunk) of the corpus callosum, which interconnects the two cerebral hemispheres, is prominently displayed in this coronal section. Superior to the corpus callosum is the cingulate gyrus, which also appears in subsequent sections. Inferior to the corpus callosum is the caudate nucleus, which appears particularly large because this section passes through the widest portion of its head (see C). The nucleus appears different in later sections because it tapers occipitally to a narrow tail (see the units that follow). The schematic lateral view (C) shows how the caudate nucleus is closely applied to the lateral ventricle and follows its concavity (shown in green). The caudate nucleus and the putamen together form the corpus striatum, whose “striation” is formed by the anterior limb of the internal capsule, a streak of white matter. The putamen still appears quite small at this level because the section passes only through its anterior tip. It becomes larger as the planes of section move further occipitally. The structures anterior to this plane consist of the cortex and white matter of the frontal lobe, both of which are easily identified. The temporal lobes, which still appear to be separate, detached structures, join the rest of the telencephalon in more occipital sectional planes (see B).

В Coronal section II

This section contains essentially the same structures as in A. The plane no longer passes through the head of the candate nucleus, instead passing through its slender body. The inferior horn (temporal horn) of the lateral ventricle appears as a slitlike structure and also provides a useful landmark: ventral to the inferior horn is a portion of the parahippocampal gyrus. Superior and medial to the inferior horn are the amygdalae (amygdaloid bodies, visible here for the first time; compare with D). They are bordered by the uncus, which is the hook-shaped anterior end of the parahippocampal gyrus. The internal capsule, which pierces the corpus striatum, appears considerably thicker in this plane than in A. The temporal lobe has merged at this level with the rest of the telencephalon, and the insular cortex is clearly visible.

C Relationship between the caudate nucleus and lateral ventricle.

Left lateral view.

D Amygdala

Right lateral view.

20.2 Coronal Sections: III and IV

A Coronal section III

The inferior (temporal) horn of the lateral ventricle appears somewhat larger in the plane of this section. In the ventricular system, we can now see the floor of the third ventricle (see B) and the surrounding hypothalamus. The thalamus cannot yet be seen, as it lies slightly above and behind the hypothalamus. The anterior commissure appears in this plane as does the globus pallidas, which consists of a medial and a lateral segment. The large descending pathway, the corticospinal tract, passes through the internal capsule, which has a somatotopic organization. The genu of the internal capsule transmits axons for the pharynx, larynx, and jaw. The course of these axons is shown schematically in C (the fornix appears in D).

В Ventricular system

Left lateral view.

C Course of the pyramidal tract in the Internal capsule Left anterior view.

D Coronal section IV

The division of the globus pallidus into medial and lateral segments can now be seen clearly. This section displays the full width of both the inferior horn of the lateral ventricle and the claustrum (believed to be important in the regulation of sexual behavior). While the plane in A passed through the anterior commissure, this more occipital plane slices the mammillary bodies (see E). Pathological changes in the mammillary bodies can be found during autopsy of chronic alcoholics. The mammillary bodies are flanked on each side by the foot of the hippocampus. An important part of the limbic system, the mammillary bodies are connected to the hippocampus by the fornix (see F). Due to the anatomical curvature of the fornix, its column is visible in more frontal sections (see A), while its crura appear as widely separated structures in more occipital sections (see C, p. 299). The septum pellucidum stretches between the fornix and corpus callosum, forming the medial boundary of the lateral ventricles (see A and D).

The first structure of the brainstem, the pons, can also be identified in this section.

E Midsagittal section through the diencephalon and brainstem Lateral view.

F Mammillary bodies and fornix

20.3 Coronal Sections: V and VI

A Coronal section V

The appearance of the central nuclear region has changed markedly. The caudate nucleus is cut twice by the plane of this section. Its body borders the central part of the lateral ventricle, and a small portion of its tail borders the inferior horn of the ventricle (see C and E). Because the head and body of the caudate nucleus rim the lateral aspect of the anterior (frontal) horn and the central part of the lateral ventricle, the caudate nucleus has a curved shape similar to that of the ventricular system (see C). Thus, the tail of the caudate nucleus is ventral and lateral in relation to its head and body. Panel E shows that a coronal section through the tail of the caudate nucleus cuts the occipital portions of the putamen. A section in a slightly more occipital plane may not contain any part of the basal ganglia at all (see B). The central part of the lateral horn has become much narrower due to the presence of the thalamus, visible here along with the thalamic nuclei. This is the first plane that displays the choroid plexus, which can be seen within the lateral ventricles. The choroid plexus extends from the interventricular foramen (not visible here) into the inferior horn. Because the foramen lies anterior to the thalamus, the plexus can be seen only in coronal sections that also pass through thalamic structures. Basal to the thalamus are the red nucleus and substantia nigra-, these are important midbrain structures that bulge into the diencephalon and extend almost to the level of the globus pallid us (not visible here; see B). The hippocampus indents the floor of the temporal horn, and its fimbria can be seen. This section also shows how the fibers of the corticospinal tract pass through the posterior limb of the internal capsule and continue into the cerebral peduncles and pons.

В Red nucleus and substantia nigra Midsagittal section.

C Ventricular system

Superior view.

D Coronal section VI

The caudal thalamic nuclei are well displayed in this section, bordering the lateral ventricles from below and the third ventricle from the sides. The putamen lies at a more oral level and is no longer visible in this plane (see the transverse section on p. 306). This section passes through the posterior limb of the internal capsule (see also C, p. 294) and the anterior part of the posterior commissure (see D, p. 299). The medial and lateral geniculate bodies, which are components of the auditory and visual pathways, appear as two darker nuclei that flank the thalamus on the right and left sides at the same level as the commissure (see F). The crura of the fornix can be seen between the thalamus and corpus callosum. This is the first section that passes through part of the cerebellum. Here the middle cerebellar peduncle passes laterally toward the cerebellar hemispheres.

E Topographical relationship between the caudate nucleus and ventricular system

F The diencephalon (with geniculate bodies) and brainstem Posterior view.

20.4 Coronal Sections: VII and VIII

A Coronal section VII

Among the diencephalic and telencephalic nuclei, we can still identify the thalamus and occipital portions of the caudate nucleus, which become progressively smaller in the following sections until they finally disappear (see C and p. 300). The occipital part of the hippocampus can be seen below the medial wall of the lateral ventricle. This section cuts the brainstem along the cerebral aqueduct (see C). The cerebellum is connected to the brainstem by three white-matter stalks: the superior cerebellar peduncle (mainly efferent), middle cerebellar peduncle (afferent), and inferior cerebellar peduncle (afferent and efferent). Because the middle cerebellar peduncle extends further anteriorly than the other two peduncles (note its relationship to the brainstem axis), it is the first peduncle to appear in this frontal-to-occipital series of sections (see also A, p. 296, and D, p. 297). The superior cerebellar peduncle begins on the posterior side of the pons and thus appears in a later section (see В and p. 300). There are no natural anatomical boundaries between the middle and inferior cerebellar peduncles, and therefore the latter is not separately labeled in the sections. The superficial veins were removed from the brain when this section was prepared, and only the internal cerebral veins appear in this and the following section.

В Cerebellar peduncles on the brainstem

a Posterior view, b lateral view.

C Coronal section VIII

The thalamic nuclei appear smaller than in previous sections, and more of the cerebellar cortex is seen. This plane passes through part of the cerebral aqueduct. The rhomboid fossa, which forms the floor of the fourth ventricle, is clearly visible in the dorsal part of the brainstem (see D and Ba). The quadrigeminal plate (lamina tecti) is also visible. Its smaller superior colliculi are particularly well displayed in this section, while the inferior colliculi are more prominent in the next section (see A, p. 300). The pineal is only partially visible because of its somewhat more occipital location (see D); a full cross-section can be seen in A, p. 300. The present section shows the division of the paired fornix tract into its two crura (see also D, p. 295). The hippocampus here borders on the inferior horn of the lateral ventricle on each side, bulging into its floor from the medial side (see also the previous sections and E). The hippocampus is an important component of the limbic system and is one of the first structures to undergo detectable morphological changes in Alzheimer’s disease.

D Midsagittal section through the rhombencephalon, mesencephalon, and diencephalon

E Hippocampal formation

Left lateral view.

20.5 Coronal Sections: IX and X

A Coronal section IX

The pineal gland, a control center for circadian rhythms, is here displayed in full cross-section (contrast with the previous section; see also D, p. 299). Below it lies the quadrigeminal plate, the dorsal part of the midbrain (note its relationship to the brainstem axis). The larger inferior colliculi of the quadrigeminal plate are more prominent here than in the previous section (the inclination of the brainstem gives them a more posterior location). The inferior colliculi are part of the auditory pathway, while the superior colliculi (more clearly seen in the previous section) are part of the visual pathway. At the level of the cerebellum, the vermis can be identified as an unpaired midline structure. The only cerebellar nucleus visible at this level is the dentate nucleus, which is surrounded by the cerebellar white matter. The deep cerebral nuclei are no longer visible in the plane of this section.

В Quadrigeminal plate (lamina tecti) Left posterior oblique view.

C Coronal section X

This plane presents the four cerebellar nuclei:

 Dentate nucleus (lateral cerebellar nucleus)

 Emboliform nucleus (anterior interpositus nucleus)

 Globose nucleus (posterior interpositus nucleus)

 Fastigial nucleus (medial cerebellar nucleus)

The longitudinally cut cerebellar vermis presents a larger area here than in the previous section. The fourth ventricle is no longer visible in the plane of this section.

20.6 Coronal Sections: XI and XII (Occipital)

A Coronal section XI

The plane of this section clearly shows the posterior (occipital) horns of the lateral ventricles; these appear only as narrow slits in the next section (see D). The section also illustrates once again how the posterior horn is a prolongation of the inferior (temporal) horn (see B). Between the cerebellum and the occipital lobe of the cerebrum lies the tentorium cerebelli (see C). The tentorium contains the straight sinus, which passes to the confluence of the sinuses. It is one of the dural venous sinuses that drain blood from the brain, beginning at the confluence of the great cerebral vein and the inferior sagittal sinus (removed during preparation of the falx cerebri). Because the dura is removed from the brain in the preparation of most tissue sections, the sinuses enclosed by the dura mater also tend to be removed.

В Ventricular system viewed from the left side

C The dural sinuses

Viewed from upper left.

D Coronal section XII

In the plane of this section, the posterior (occipital) horn of the lateral ventricle has dwindled to a narrow slit. The relatively long calcarine sulcus is visible in the occipital lobe of the cerebrum, and also appears in several of the proceeding sections. It is surrounded by the striate area (primary visual cortex, also called area 17 in the Brodmann brain map, p.202), the size of which is best appreciated on the medial surface of the brain (see E). More occipital sections are not presented in this chapter, as they would show nothing but cortex and white matter.

E Right striate area (visual cortex)

Medial surface of the right hemisphere, viewed from the leftside.

20.7 Transverse Sections: I and II (Cranial)

General remarks on transverse brain sections

The sections in this series are viewed from above and behind the head; i.e., the observer is looking atthe surface of the slice as it would typically appear in a brain autopsy or during a neurosurgical operation. Thus, the left side of the brain appears on the left side of the drawing. This contrasts with the image orientation in CT and MRI, where brain slices are always viewed from below; i.e., the left side of the brain appears on the right side of the image.

A Transverse section I

This highest of the transverse brain sections passes through frontal, parietal, and occipital structures of the telencephalon. Each of the two/at- его/ ventricles is bordered laterally by the body of the caudate nucleus, and medially by the trunk of the corpus callosum. The corpus callosum transmits fiber tracts which interconnect areas in both hemispheres that serve the same function (commissural tracts). When viewed in cross section, the corpus callosum appears to be interrupted by the ventricles and caudate nucleus, when, in fact, it arches over these structures, forming the roof of the lateral ventricles. The course of the tracts that pass through the corpus callosum can be appreciated by looking at a coronal section (see B).

C Transverse section II

In this section, unlike the previous one, the lateral ventricle appears divided in two. Because this section is at a lower level, it cuts the anterior and posterior horns of the lateral ventricle separately, missing the central part of the ventricle (see D). It also cuts a broad swath of the internal capsule with its genu and anterior and posterior limbs. The optic radiation, which runs in the white matter of the occipital lobe, is not labeled here because it has no grossly visible anatomical boundaries. The corpus callosum also appears divided into two parts: the genu anteriorly and the trunk more posteriorly. This apparent division results from a second curvature of the corpus callosum at its genu (“knee”), where it is anteriorly convex. The diagram in E demonstrates why this section passes successively through the genu of the corpus callosum, the septum pellucidum, the body of the fornix, and finally the trunk of the corpus callosum. The septum pellucidum forms the anteromedial wall of both lateral ventricles. The septum itself contains small nuclei. Sections of the thalamic nuclei (ventral lateral, lateral dorsal and anterior nuclei) are also visible along with the putamen and caudate nucleus. The head and tail of the caudate nucleus appear separately in the section (see also fcj.306) The putamen, caudate nucleus, and intervening fibers of the internal capsule are collectively called the corpus striatum.

D Lateral view of the ventricular system

E Corpus callosum and fornix

20.8 Transverse Sections: III and IV

A Transverse section III

The lateral ventricles communicate with the third ventricle through the interventricular foramina (of Monro). They are located directly anterior to the thalamus (see D, p.305, and A, p. 296). The nuclei of the telencephalon make up the deep gray matter of the cerebrum. The spatial relationship between the caudate nucleus and thalamus is illustrated in B. The caudate nucleus is larger frontally, and the thalamus larger occipitally. While the caudate nucleus and putamen of the motor system belong to the telencephalon, the thalamus of the sensory system belongs to the diencephalon. This transverse section passes through the caudate nucleus twice due to the anatomical curvature of the nucleus. This is the first transverse section that displays the globus pallidus, part of the motor system. The insular cortex is seen with the claustrum medial to it. The crura of the fornix are seen as posterior to the thalamus (see also E, p. 305). They unite at a slightly higher level to form the body of the fornix, which lies just below the corpus callosum and was visible in the previous section (see C, p. 305). The course of the internal capsule is visible in both this section and the last.

В Spatial relationships of the caudate nucleus, putamen, thalamus, and lateral ventricles

Left anterior oblique view.

C Transverse section IV

The nuclei shown in the previous section here appear as a roughly circular mass at the center of the brain, surrounded by the gray matter of the cerebral cortex, also called the pallium (“cloak”) for obvious reasons. The choroid plexus is here visible in both lateral ventricles. This section cuts the occipital part of the corpus callosum, the splénium, as well as the basal portion of the insular cortex. The insula is a cortical region that lies below the surface and is covered by the opercula. The insular cistern should be used as a reference point, e.g., when comparing this section to A and D.

20.9 Transverse Sections: V and VI (Caudal)

A Transverse section V

Structures visible in this section include the cerebral aqueduct, the basal part of the third ventricle (see also B, p.294), and the optic recess. While the third ventricle is slitlike at this level, the section cuts a very large area of the ventricular system where it opens into the two posterior horns. This is the first transverse section that displays the midbrain (mesencephalon), passing through its oral portion (note: terms of location and direction refer to the brainstem axis, see p. 198). The cerebral peduncles (crura cerebri), the substantia nigra, and the superior colliculi of the quadrigeminal plate can also be seen. Visible structures of the diencephalon in this plane include the medial and lateral geniculate bodies (appearing only on the right side, see B) and the optic tract, which is an extension of the diencephalon.

Note: closely adjacent structures in the brain may belong to different ontogenetic regions. For example, the medial and lateral geniculate bodies are part of the diencephalon, while the superior and inferior colliculi (the latter is not visible), which make up the quadrigeminal plate, are part of the mesencephalon. It should be recalled, however, that the lateral geniculate body and superior colliculus are part of the visual pathway while the medial geniculate body and inferior colliculus are part of the auditory pathway.

C Transverse section VI

The structures that occupy the largest area at this level are the telencephalon, the medial portions of the mesencephalon, and the cerebellum. The nuclei located on the median aspect of each frontal lobe of the telencephalon are the amygdalae. The lower part of the section cuts the calcarine sulcus with the surrounding visual cortex. This section also passes through the choroid plexus of the lateral ventricles, whose posterior and inferior horns are displayed. Important structures of the mesencephalon are the substantia nigra and red nucleus, both of which are part of the motor system. The mammillary bodies are part of the diencephalon and are connected by the fornix (not visible in this section) to the hippocampus, which is part of the telencephalon. The mammillary bodies lie in the same horizontal plane as the hyppocampus and the same coronal plane as its pes (foot). These relationships result from the curved shape of the fornix (see D). More transverse sections at lower levels would supply little additional information on the cerebrum, and so our series of transverse sections ends here. The brainstem structures lying below the mesencephalon are displayed in a separate group of sections (see p. 234, 235).

20.10 Sagittal Sections: I-III (Lateral)

A Sagittal sections I—III

Left lateral view. The plane of section a passes through the inferior (temporal) horn of the lateral ventricle; the more medially situated posterior (occipital) horn is seen in b and c (see C, p. 296 for relative position of both horns). The amygdala, which is directly anterior to the inferior horn, lies in the same sagittal plane as the parahippocampal gyrus (a-c; see also C, p. 309). The internal capsule can also be seen in sections a-c; the long ascending and descending tracts pass through this structure. The most lateral section (a) offers the only view of the insular cortex, a part of the cerebral cortex that has sunk below the surface of the hemisphere (compare with the coronal sections on p. 293 and the following pages). The putamen, the most laterally situated among the basal ganglia of the telencephalon (see also A, p. 296) is also found in a, but appears larger in the more medial sections (b, c). A portion of the claustrum can be seen ventral to the putamen (a), although most of the claustrum is lateral to the putamen (see A, p.297) and outside the plane of the section. Section bjust cuts the tail of the caudate nucleus, which is situated more laterally than its head and body (see also D, p. 279). The most medial section in this series (c) cuts the calcarine sulcus (see p. 312) and the lateral geniculate body which lies at the edge of the thalamus. The lateral segment of the globus pallidus can also be seen (c): the segments of the globus pallidus are actually medial to the putamen (see D, p.295), but can be visualized here due to their concentric arrangement.

20.11 Sagittal Sections: IV-VI

A Sagittal sections IV-VI

Left lateral view. The dominant ventricular structures in all three of these sections are the anterior horn and central part of the lateral ventricle (the junction with the laterally situated posterior horn appears only in a). The corpus callosum, which connects functionally related areas of the two cerebral hemispheres (commissural tract), can be identified in the cerebral white matter although it is not sharply delineated (a-c). As the sections move closer to the cerebral midline, the putamen grows smaller while the caudate nucleus becomes increasingly promiment (a-c). These two bodies are known collectively as the corpus striatum, and their characteristic striations are seen particularly well in a (the white matter that separates the gray-matter streaks of the corpus striatum is the internal capsule). The previous sagittal sections showed only the lateral segment of the globus pallidus (see p. 310), but its medial segment is displayed in both a and b. As the globus pallidus disappears and the putamen becomes less prominent, the nuclei of the medially situated thalamus become visible below the lateral ventricle (c; the subthalamic nuclei include the anterior, posterior, and lateral ventral nuclei of the diencephalon). The location of the thalamus explains why it is sometimes referred to as the dorsal thalamus. Section c also shows the substantia nigra in the mesencephalon (below the diencephalon), the inferior olivary nucleus in the underlying medulla oblongata, and the dentate nucleus of the cerebellum. The ascending and descending tracts previously visible only in the internal capsule can now be seen in the pons, part of the brainstem (c, corticospinal tract). The only visible portion of the fourth ventricle, barely sectioned in c, is its lateral recess.

20.12 Sagittal Sections: VII and VIII (Medial)

A Sagittal sections VII and VIII

Left lateral view. This section (a) is so close to the midline that it passes through the principal paramedian structures: the substantia nigra, the red nucleus, and one each of the paired superior and inferior colliculi. The pyramidal tract (corticospinal tract) runs in front of the inferior olive in the medulla oblongata. A complete sagittal section of the corpus callosum is displayed, and most of the fornix tract is displayed in longitudinal section (b). The cerebellum has reached its maximum extent and forms the roof of the fourth ventricle (b). A portion of the septum pellucidum, which stretches between the fornix and corpus callosum, is also displayed.

When the brain is removed, the pituitary gland, which appears in b, remains in the sella turcica; i.e., it is always torn from the brain at its stalk when the brain is removed.

В Principal structures in the serial sections

The major structures seen in the serial sections are here assigned to their corresponding brain regions. Within each region, the structures are listed from most rostral to most caudal.

Telencephalon (endbrain)

Diencephalon (interbrain)

• External capsule

• Lateral geniculate body

• Extreme capsule

• Medial geniculate body

• Internal capsule

• Pineal gland

• Claustrum

• Pulvinar of thalamus

• Anterior commissure

• Thalamus

• Amygdala

• Optic tract

• Corpus callosum

• Fornix

• Mammillary body

• Globus pallidus

Mesencephalon (midbrain)

• Cingulate gyrus

• Cerebral aqueduct

• Hippocampus

• Quadrigeminal plate (lamina tecti)

• Caudate nucleus

- Superior colliculus

• Putamen

- Inferior colliculus

• Septum pellucidum

• Red nucleus

• Substantia nigra

• Cerebral peduncle (crus cerebri)